This invention relates to a tone signal processing device employing a digital filter used in an electronic musical instrument and, more particularly, to a tone signal processing device of a type which realizes timewise change of filter characteristics by preparing timewise changing filter coefficients by an interpolation operation capable of variably controlling the rate of change in accordance with tone color of a tone.
An electronic musical instrument realizing timewise change of a tone color by timewise changing filter coefficients of a digital filter provided for controlling a tone signal is disclosed in U.S. Pat. No. 4,738,179. In this electronic musical instrument, filter coefficients are stored in a memory in correspondence to a predetermined time frame and, at each time frame, a corresponding filter coefficient is read from the memory and supplied to a digital filter. Thus, filter characteristics are changed frame by frame by changing filter coefficients frame by frame. In conventional electronic musical instruments, for variably controlling filter characteristics for a tone color control in accordance with key touch or key scaling (variable control corresponding to tone pitch), filter coefficients had to be prestored in a memory for each realizable filter characteristics.
U.S. Pat. No. 4,907,484 discloses a device in which a larger number of filter coefficients than filter coefficients stored in a memory are generated by an interpolation operation on the basis of filter coefficients read from the memory whereby filter characteristics having abundant variation can be realized without increasing the amount of filter coefficients stored in the memory. This device performs a tone color control corresponding to key scaling but this is key scaling of a static tone color and not key scaling of dynamic change of tone color, i.e., the rate of timewise change of tone color. When, for example, an interpolation operation is performed on the basis of a first filter coefficient and a second filter coefficient, interpolation coefficient is changed with lapse of time during a predetermined time frame and the filter coefficient obtained by this interpolation operation thereby changes with lapse of time from the first filter coefficient to the second filter coefficient. According to the prior art device, the value of the first filter coefficient and the second filter coefficient which are used as the basis of the interpolation operation can be key scale controlled but the rate or time length of timewise change thereof cannot be key scale controlled.
It is known that in a natural musical instrument, not only its steady-state tone color but the rate of timewise change of the tone color change in accordance with its tone pitch. The prior art device is insufficient in that it can key scale control the steady-state tone color but it cannot key scale control the rate of timewise change of the tone color.